Optical films such as those used in liquid crystal displays, glazings, and other laminates and layered products demand high light transmissivity and ultra-clean appearance. Defects such as particles, non-planar topography, and disproportionate degree of contact (sometimes referred to as “wet-out”) that are present in an optical film(s), however, can result in undesirable malappearances, and can be detrimental to the light transmission, the brightness enhancement function or clarity of the product. These defects can be, in part, a result of static charges that are introduced by manufacturing, converting or assembly processes.
For example, static charges can result from a tape (e.g. masking) or other film that is quickly pulled or peeled away from the target substrate/film during processing. These static charges can subsequently attract particles that may be near the surface of a film. Particles that eventually land or become anchored on the film can lead to unwanted light blockages, refracting, or absorbance, depending on the film's original purpose. A non-planar topography can be the result of non-uniform shrinkage, warping, or expansion of a film, particularly when an area of the film is pinched or mechanically held in place while movement or creep occurs with another portion of the film. Another cause, however, may be static charges that can create the pinched or stationary area, causing binding between film layers and consequently lead to non-uniform or non-synchronized film changes. The optical defect known as the “wet-out” phenomenon can occur when differences in optical transmission exist between two regions, or when interference patterns such as “Newton's rings” are observed. (The defect is minimally detectable when the wet-out is uniform throughout a film product.) Static charges can contribute to non-uniform attraction of particular areas between two layered films, causing wet-out.
Conductive compositions have been developed since the introduction of conductive polymers such as polyethylenedioxythiopene (PEDT). Some conductive polymers are dispersible in water and alcohol, rendering them a popular choice for conductive coating compositions. Applying these compositions onto films (e.g. on the surface) are known to impart anti-static properties even in the absence of significant ambient humidity. “Anti-static” or static dissipative materials with these surface coated conductive compositions are typically characterized as having a surface resistivity or less than about 1×1012 ohms/square and a static decay time of less than about 2 seconds.
Some conductive compositions, however, may have limited light transmissivity, likely due to their highly colored nature, and therefore have limited use in certain optical film products that require high transmissivity and clarity, such as optical-grade display films. Moreover, some polymeric coatings can be susceptible to mechanical abrasion and other undesirable or optically disruptive effects when left unprotected. Such mechanical disruptions can be quite detrimental for optical articles. For example, a smudge or scratch on a polymeric coating can result in an undesirable effect when the article is a computer display.
Static dissipative materials have been developed for industries such as carpets, electronics, (e.g. IC wafers, sensors, semiconductors) and packaging. Currently, materials developed for these applications rely on conductive compositions coated onto a surface of a material and left exposed to the environment, or materials that have anti-static agents within its composition, such as by extrusion of a bulk composition pre-blended with an anti-static agent, or by penetration, absorption, or migration of an antistatic agent into the composition. There are antistatic agents that require some amount of water (humidity) to be effectively static dissipative. These are typically the ionic type of antistatic agents which rely on ionic mobility for the dissipative mechanism. Their effectiveness, however, can be reduced as a function of humidity—i.e. as relative humidity decreases, the static dissipative ability decreases. Typically, at relative humidities less about 20% RH, ionic anti-static agents may not be effective.
It is therefore desirable to provide an optical article that can be static dissipative even when a conductive or anti-static coating has been buried (e.g. protected) by non-conductive material(s), and also be static dissipative in lower humidity environments. Optical articles that can be both static dissipative and still capable of maintaining desired levels of light controlling ability are also needed. Processes for manufacturing optical-grade constructions and articles with minimal defects caused by static charges would be beneficial.